How to filter multiple file-extensions to limit which files display in an operator's file-selector?
In the operator's filter_glob property, (which you've already defined), you can define multiple extensions using the ; separator, eg:

filter_glob = StringProperty(
        default="*.png;*.jpg;*.jpeg;*.bmp;*.exr",
        options={'HIDDEN'},
        )


If you want to make a list of all extensions Blender supports you can use. bpy.path.extensions_image eg:

"*" + ";*".join(bpy.path.extensions_image)